It is often desirable to provide a means to phase shift a signal, such as a radio frequency (RF) carrier signal. For example, relative phase shifts between simulcast signals may be used to provide radiation pattern shaping and beam forming from an antenna. Moreover, it may be desirable to provide for adjusting or selecting such relative phase shifts in order to provide steerable antenna beams.
In a beam-forming architecture where the antenna is mechanically fixed, phase shift control allows radiation patterns to be created which can handle multiple simultaneous cellular phone calls, or improvements to radar systems which require spatially agile antennas. In order to make an antenna pattern electronically changeable, a phase shifter is, for example, often used. Such phase shifters are used to change the relative phase between the individual radiating elements of an antenna, while keeping frequency and amplitude fixed. The electronics that are suitable for phase shift control can fall into many different categories, depending on design parameters, signal requirements, and the like. Accordingly, adjustable phase shifter designs are realized through various technologies, each with its own performance advantages and disadvantages. For example, a ferrite core phase shifter requires a lot of space, is very heavy, and experiences hysteresis (a previous command must be erased before another command is written to the device). They also consume large amounts of DC power, but can handle high signal power.
A quadrature modulator is another effective way to change the phase of a signal, but such devices are very limited in terms of their input power-handling capability. Therefore, it exacerbates the design of the network around it, in order to control the input signal level, while accommodating a large variation after the modulator. Using a quadrature modulator to phase shift a signal requires applying DC voltages to its intermediate frequency (IF) inputs, but to do so requires learning this behavior, and then providing a lookup table of phase versus IF voltage in order to use the device. However, quadrature modulators are typically compact (often realized as integrated chip sets) and provide substantially unlimited phase resolution.
Another type of phase shifter, xe2x80x9cloaded line,xe2x80x9d uses selectable transmission lines to reactively load, i.e., with capacitance or inductance, to change the insertion phase of the device. These loaded line phase shifters can have high insertion lossxe2x80x94a significant portion of the input signal power is lost. They can also have low linearity, measured as a figure of merit called intercept performance, and therefore distort a signal even at low power levels. Accordingly, although loaded line phase shifters are generally compact, they are lossy, have poor dynamic range, are typically expensive, and employ high-risk designs.
Finally, there are switched-line phase shifters, which allow selection of one signal path or another, made of various lengths of transmission lines, which effectively changes the electrical length, and therefore the phase, of the composite path. Switched line phase shifters are generally limited to relatively narrow bandwidths because they utilize transmission line lengths designed for one particular frequency. As other frequencies are utilized therewith, the phase shifter will appear electrically longer or shorter. Dynamic range is limited by the linearity of the switching devices, and only quantized states are available. Also, they typically occupy large areas, and can have poor isolation. However, switched line phase shifters employ low risk designs that are inexpensive, have low loss, and provide adequate RF power handling.
From the above, it can be seen that there is a trade off between several designs, and operational factors, such as insertion loss, intercept performance (which controls linearity of the signal), bandwidth, and the like. Cost is also a factor, as is the number of separate phase shifters required and available space. For example, in a system which provides multiple narrow antenna beams in order to provide complete 360xc2x0 coverage about a cellular base transceiver station, not only are multiple phase shifters required to form a single beam, but multiple sets are required to form multiple beams. Thus, factors such as cost and size can become critical,
Accordingly, there is a need in the art for providing phase shifters which provide a desired degree of phase resolution, with low insertion loss, high linearity, good dynamic range, low power consumption, small size, low risk, and low cost.
These and other objects, features and technical advantages are achieved by a system and method employing a phase shifter design which uses a switched line structure on a printed circuit board, or other support structure. Systems operating over a relatively narrow bandwidth, such as 869 to 894 MHz associated with the cellular transmit band for example, are very acceptable for the application of this form of phase shifter.
In order to provide fine resolution, i.e., an ability to provide for relatively small incremental changes in phase shift, over a broad range, a preferred embodiment of the present invention defines multiple switched line sections with increasingly smaller lengths. Preferably, the lengths are designed to be cardinal states of a digital control word. Accordingly the line lengths are successively divided in half, i.e., if the greatest line length in the switched line phase shifter is 180xc2x0 (xcex/2, where xcex is the wavelength in the dielectric media of the design frequency), the next is 90xc2x0 (xcex/4), the next is 45xc2x0 (xcex/8), and the next is 22.5xc2x0 (xcex/16), until the finest resolution desired is reached. Accordingly, if a specific phase shift is desired, a control system may be operated to activate, or command, certain sections of the phase shifter, corresponding to a specific digital control word, in order to achieve a net phase shift.
It should be appreciated that the above described phase shifter could require a substantial amount of space, such as a large surface area on a circuit board, in order to provide the line lengths necessary to achieve both the desired range of phase shift and a desired resolution. Moreover, there is the likelihood that placing a number of such switched lines, either from a single phase shifter of multiple segments or multiple such phase shifters, in near proximity, such as on a circuit board, may result in adjacent sections being electrically coupled to each other. In other words, poor signal isolation can be a result if steps are not taken to control this phenomenon.
The preferred embodiment phase shifter is small in order to take up less space on a circuit board, or other supporting structure, contrary to generally accepted practice. Additionally, the preferred embodiment phase shifter provides high linearity in order for it to handle higher power signals. Likewise, the phase shifter of the present invention preferably exhibits relatively low loss. Preferably, the phase shifter of the present invention provides phase shifting with low DC power consumption, which becomes very important in circumstances where a relatively large number of phase shifters are utilized, such as when many simultaneous antenna beams are generated.
In achieving the above described attributes, the phase shifter of a preferred embodiment of the present invention utilizes microstrip and/or stripline transmission lines of selected lengths to provide switched line lengths which are small in size, low loss, inexpensive, and provide good isolation of signals. A stripline is an RF transmission line disposed, or xe2x80x9csandwiched,xe2x80x9d between two ground planes, most often within a dielectric media. Accordingly, striplines may be buried within the strata of a circuit board and, thus, can utilize the same area in one dimension (e.g., the x-y plane), while being separated in another dimension (e.g. the z-axis), to occupy a small amount of space per phase shifter. Of course, other structure for providing phase shifts, such as surface acoustic wave (SAW) devices, different forms of transmission media providing different rates of propagation, and the like, may be utilized, i.e., switchably selected, according to the present invention. Moreover, these various structures may be separated in various planes as described above.
Utilizing multiple different layers to minimize the space occupied by a phase shifter suggests the use of layer changing transitions, such as layer changing vias in a printed circuit board. However, it should be appreciated that in any high-frequency transmission path, it is desirable to maintain constant impedance to avoid such undesirable results as signal reflection, higher insertion loss, and the like. Accordingly, stratum or layer changing vias adapted to maintain constant impedance are preferably utilized in providing transmission paths of the present invention. Primarily, a layer changing via as used in a circuit board is a hole drilled in the dielectric substrate, and plated to connect transmission lines disposed on various layers of the substrate. Preferably, the constant impedance layer changing vias of the present invention are as described above, and adhere to a specific design geometry to maintain constant impedance. Of course, layer-changing vias of the present invention may be widely used in structures other than the circuit board of the preferred embodiment. For example, vias adapted according to the present invention may be utilized in stratified structures such as monolithic integrated circuits, such as may be provided by properly doping areas of the monolithic structure to define stratum-changing vias.
Using the preferred embodiment vias adapted to provide consistent impedance, the preferred embodiment striplines may be disposed on multiple layers of a multi-layer printed circuit board and provide electrically transparent transitions from one layer to another. For example, utilizing the vias of the preferred embodiment, changing a transmission line from the surface of the circuit board, such as a microstrip transmission line, to a buried line, such as a stripline transmission line, the layer changing via is adapted to appear electrically transparent as possible to the transmitted signal. Accordingly, a transmission line, although changing layers in a circuit board and/or changing between forms of transmission line, continues to behave like one of constant impedance, such as 50 Ohms.
Often the substrate, in addition to having transmission lines disposed thereon, also includes grounding layers, such as ground plane layers disposed on the surfaces and intervening layers or strata, of the substrate. Often these grounding layers are utilized in providing isolation between signals and a DC-reference for applied voltages. They are also commonly used as a ground reference for RF signals, to define the impedance of the transmission lines. Accordingly, there is a clearance between the conductor associated with the transmission of RF signals, and the metalized ground plane, such as the above mentioned stripline and microstrip structures, and the layer changing vias.
The dimension of the clearance between the conductor of a via and the ground plane will define the capacitance and inductance, or reactance, of the layer changing via. For example, if the ground clearance is too small, the via will appear capacitive. Contrariwise, if the ground clearance is too large, the via will be inductive. Accordingly, if the ground clearance of a layer changing via is not properly selected, the via will appear to the signal as an instantaneous change in system impedance, which looks reflective to an incoming RF wave that is propagating through the transmission line. Accordingly, a preferred embodiment of the present invention utilizes circuit boards having layer changing vias employing predetermined ground plane clearances, such as between the plated pads where the stripline and/or microstrip connects to the vias, and the surrounding ground plane, in order to provide consistent impedance in the signal path. Additionally, the size, or diameter, of the vias of the preferred embodiment are selected so as to provide consistent impedance in the signal path.
Utilizing design rules, empirically developed in attaining the present invention, it is possible to provide a change in a transmission line between layers without upsetting the system impedance, Such as with the above mentioned preferred embodiment vias. Therefore, the preferred embodiment of the present invention, utilizing such design rules to provide vias to move transmission lines between various layers of dielectric such as in a printed circuit board, can compact the size of the phase shifter. Moreover, the layer changing vias, designed empirically to maintain system impedance, improve the return loss associated with the vias. Experimentation has revealed the worst case for the preferred embodiment device described herein, tested over the entire phase constellation, is approximately xe2x88x9214 dB, with a large percentage of the states being anywhere from xe2x88x9225 dB to xe2x88x9230 dB. Accordingly, the preferred embodiment phase shifter may be integrated into an RF cascade configuration without upsetting the system impedance, loss, and passband ripple of the composite circuit.
A preferred embodiment of the present invention utilizes PIN diodes, such as surface mount PIN diodes, in providing switching of the various transmission lines of the phase shifter. These diodes operate in the on-biased region, with very little DC bias current. Preferably two such diodes are used per switchable transmission line section to allow selection of a phase shift associated with the transmission line, by properly biasing the diodes.
For example, where the eight different sections, and therefore different phase shift, switched lines of a preferred embodiment phase shifter are utilized, each having a zero phase shift reference line associated therewith for selection when the phase shift of a particular section is not desired, 32 such diodes would be provided. However, only 16 of these 32 diodes would be ON at any one time. A preferred embodiment PIN diode commercially available requires only 3.5 milliamperes (mA) of bias current. Accordingly, the above example biasing of 16 diodes per phase shifter would utilize approximately 56 mA to operate one phase shifter. With a 5-volt power supply, this would require approximately 17.5 milliwatts per section, or 0.28 Watts total consumption for the preferred embodiment, which is very low for a commercial design. Moreover, the PIN diodes of the preferred embodiment have very low insertion loss with just a diminutive amount of bias current. Therefore, total loss through the above described eight sections of a preferred embodiment phase shifter results in four to five decibels (dB) of loss.
A preferred embodiment of the phase shifter exhibits high linearity, measured as Third-Order Intercept (IP3), using the aforementioned PIN diodes. A preferred embodiment PIN diode operates with an output IP3 of +40 dB above a milliwatt (+40 dBm). Cascading eight sections of the above described phase shifter with the low insertion loss, results in a total output IP3 of +25 dBm.
A preferred embodiment of the phase shifter uses eight of the above mentioned switched line sections, the smallest resolution of which is 2.8xc2x0 for RF frequencies of approximately 894 MHZ. Utilizing the above described transmission lines disposed on different layers, these switched lines may be deployed in a small area, such as approximately four square inches (in.2) of circuit board. Accordingly, this preferred embodiment phase shifter can provide 256 states with a resolution of slightly less than three degrees, occupying a surface area of about four in.2.
It should be appreciated that the parts of the preferred embodiment, utilizing PIN diodes, stripline and/or microstrip transmission lines, biasing resistors, and tuning inductors and capacitors, are negligible in cost. For example, at today""s prices it is estimated that the cost of an entire phase shifter is well under $10 and most likely in the $6 range.
Accordingly, it is a technical advantage of the present invention that phase shifting is provided with a desired degree of resolution, with low insertion loss, and/or with a high degree of RF linearity. Moreover, a further technical advantage is realized by the present invention providing any or all of the above features in the art for phase shifters in a relatively small package. A still further technical advantage of the present invention is that a phase shifter having the above features may be provided inexpensively.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.